The five year survival rate among children diagnosed with acute lymphoblastic leukemia (ALL) has surpassed 90%. Unfortunately, cure is not without consequences. ALL survivors are at elevated risk for obesity, low bone mineral density, cardiomyopathy, peripheral neuropathy, and altered psychosocial functioning. Research is critically needed to provide survivors with tools to improve their long term health. Because children with leukemia are likely to adopt a sedentary lifestyle that persists long into survivorship, and because a sedentary lifestyle may perpetuate or exacerbate many of the complications experienced by ALL survivors, a portable and generalizable intervention that promotes physical activity and impacts fitness and biomarkers of cardiometabolic health in the first year after treatment may be of particular benefit. This proposal is designed to access the resources of the Children's Oncology Group (COG) and complements ongoing therapeutic protocols. It will draw upon the existing infrastructure and expertise of the COG to recruit a sample of children as they complete treatment for childhood ALL. The primary goals of this study are to evaluate the effects of a rewards-based physical activity intervention, delivered via an interactive website for children with ALL, during a six month period beginning any time during the first three months after the end of curative chemotherapy, on fitness, biomarkers of cardiometabolic health, inflammation, adipokine status, quality of life and school attendance. We also plan to evaluate if the effects of the intervention on markers of cardiometabolic health are mediated by changes in fitness. To achieve this goal we have designed a two-arm, prospective, randomized study with a rewards- based intervention group (N=192) and a control group (N=192). The control group will receive educational materials encouraging physical activity, including information about activity modifications for common neuromusculoskeletal deficits (e.g. neuropathy, weakness), and an activity monitor to record physical activity levels. The intervention group will receive the same educational materials, the activity monitor, and access to a rewards-based interactive website designed to motivate increased levels of physical activity. Intervention group participants will receive points based on physical activity levels. Points are redeemed for small gift cards ($5.00) during the course of the study. Based on our pilot study, the average child/adolescent will earn eight gift cards. Individual physical activity levels are uploaded to the website (with acknowledgement of the data upload for control group participants, and with an interactive video game-type dashboard for intervention group participants when the activity monitor is connected to a computer). Rewards and encouragement are provided by the website based on the intervention participants' measured physical activity levels. Children who are initially less active still earn points as the rewards system is based on individual baseline physical activity. Evaluation time points are planned at baseline, after intervention, and six months and one year after the intervention is completed.